1. Field of the Invention
The present invention generally relates to an object detecting device for detecting the fact that an object is positioned in a predetermined area, and particularly relates to an object detecting device arranged in such a manner that a magnetic-flux generating source and a detector portion are disposed separately from each other in a predetermined area so that when an object at least partly having a magnetic substance or magnetic-flux permeable material such as iron is positioned above the area, the magnetic flux is transmitted to the detector portion through the magnetic substance or magnetic-flux permeable material such as iron of the object so that the presence of the object is detected on the basis of a change in the detected magnetic flux.
2. Description of the Prior Art
In order to park plurality of objects, for example, vehicles such as cars, motor bicycles, and so on, each of which contains a magnetic substance or magnetic-flux permeable material such as iron, in a parking zone, vehicle detecting devices are provided respectively in parking sections of the parking zone so as to detect presence of parked vehicles. Such a vehicle detecting device is made by using light rays, an ultrasonic wave, or a microwave, or by using a buried loop coil. More specifically, a pole provided with a light emitting device or an oscillator for generating an ultrasonic wave or a microwave is planted in the vicinity of an inlet of each of the parking sections. Alternatively, a light emitting device or an oscillator for generating an ultrasonic wave or a microwave is provided on the ceiling of the parking zone so as to emit light rays, an ultrasonic wave, or a microwave toward each of the parking sections. Existence of a vehicle is detected by detecting shielding of light rays due to presence of the vehicle in the case of using the light rays, or by catching a reflection wave which changes by presence of the vehicle in the case of using an ultrasonic wave or a microwave. Alternatively, a loop coil is buried in the ground in each of the parking sections so that presence of a vehicle is detected by detecting a change in inductance of the loop coil caused by presence of the vehicle in the section.
However, the light rays from the light emitting device or an ultrasonic wave or a microwave from the oscillator spread in proportion to the distance from the light emitting device or from the oscillator to a vehicle in a target section, so that there is a possibility that not only the vehicle in the target section but any other vehicles in other sections adjacent to the target section may be detected by the light rays or by the ultrasonic wave or microwave when the ceiling of the parking zone is high. In order to prevent such erroneous detection from occurring, it is necessary to focus or condense the light rays, the ultrasonic wave or the microwave. If the light rays, the ultrasonic wave or the microwave are focused or condensed so narrowly, however, there occurs another defect that also a human body may be detected. In the method of catching a reflection wave, on the other hand, an ultrasonic wave or a microwave may be reflected in an unexpected direction depending on the curved surface of a car body or the angle of a front glass so that sometimes it becomes impossible to catch the reflection wave. Further, when a light emitting device or an oscillator is incorporated in a pole at a lower portion thereof, the light emitting device or the oscillator is broken frequently by mischief or the like. Moreover, in the case of detection by using light rays, since it is necessary to set a photo-detector portion on the floor surface even if the light emitting device is attached on a high position such as a ceiling or the like to prevent mischief, there is a possibility that the photo-detector portion is damaged or covered with dust to thereby operate erroneously. Further, in the case of detection by using a loop coil, there is a defect that a wide area is occupied by the loop coil because the loop coil is required to stretch over an area of about 1.5.times.l.5 m.sup.2 at minimum, although the loop coil can be protected from such mischief because it is buried in the ground.